This invention relates to an image processing apparatus and method and, more particularly to a color image processing apparatus and method which is helpful in guarding against the production of counterfeiting of bank notes, securities, and the like.
Conventionally, various techniques are implemented in a color image processing apparatus, such as a full-color copying machine, in order to prevent the counterfeiting of bank notes and securities. One technique is such that a pattern, which is unique to each image formation apparatus but imperceptible to human eyes, is added on the image information at a predetermined modulation amount in order to identify the image processing apparatus used for counterfeiting. Such technique is disclosed in U.S. Ser. No. 07/856,996, U.S. Pat. No. 5,257,119 U.S. Ser. No. 07/996,692, U.S. Pat. No. 5,457,540 and U.S. Ser. No. 08/009,735. In a case where bank notes or securities have been forged by such color image formation apparatus, if a unique pattern is read and identified by a reading apparatus capable of detecting a predetermined wavelength range represented by the color printed on a counterfeit, the image formation apparatus used for the counterfeiting can be identified effectively be traced, Thus a counterfeiter can.
In the conventional technique, a pattern is added in a neutral tint so that the pattern itself cannot be read; however, fog appears on an image because the pattern exists on the image boundary where an image begins on a printing paper. Particularly, the problem is evident, in a boundary between an image portion and a non-image portion, an add-on pattern stands out resulting in the deterioration of image quality.
Furthermore, when the image formation characteristic of the color image formation apparatus is changed, the add-on unique pattern cannot be read and/or the image may not be formed properly. That is, the added unique pattern may not be able to be read since the change of characteristics are not reflected in the image density at a highlight portion of the image (in a low density range). This is due to the dependency on the tone characteristic of the color image formation apparatus. In contrast, in the density range where the contrast is strong, the add-on unique pattern becomes visable.
More particularly, in an electrophotographic process copying machine, when an image density reproduction capability decreases by deterioration of a photoreceptor and the image density cannot be fully expressed, a drawback occurs in that an add-on unique pattern cannot be reproduced and read as a difference of the image densities.
When the image density reproduction capability increases and the image density is excessively expressed, the drawback is such that the add-on unique pattern becomes visable because the difference between the image density of the add-on unique pattern and that of the original image is significantly large; thus the image is not accurately reproduced.
Furthermore, when a recording density of the full-color copying machine is increased to duplicate specific originals such as bank notes, stamps, securities, and postage stamps, since the color reproduction is deteriorated and character lines are emphasized, the drawback occurs in that the unique pattern of the image formation apparatus-is difficult to reproduce. In the low density range in particular, the unique pattern is difficult to reproduce and read. In contrast, in the intermediate density range, the added unique pattern becomes visable.